This invention generally relates to sputtering devices, and more particularly to sputtering devices employing magnetic field forming means for enhancing the sputtering of select materials upon a substrate. As will be more fully appreciated from the discussion which follows, this invention involves an improvement to such sputtering devices which renders them particularly useful in coating substrates which are susceptible to deterioration from excessive heat.
An exemplary embodiment of a sputtering device to which this invention relates is a planar magnetron. Briefly stated, a planar magnetron is a sputtering device comprising a cathode, a rectangular planar target connected to, and maintained at, substantially the same high negative potential applied to the cathode, and a plurality of permanent magnets or electromagnets positioned proximate a surface of the target. Said magnets are generally arranged in a closed-loop, or racetrack, configuration such that the lines of magnetic flux produced by said magnets result in the formation of a magnetic field which projects outwardly from the opposite surface of the target and generally towards the substrate to be coated. Because of the closed-loop configuration of said magnets, the magnetic field produced thereby is also of a closed-loop, or racetrack, configuration.
Planar magnetrons sputter efficiently because the magnetic field acts to trap therewithin certain of the electrons emitted from the target. These electrons, under the influence of the magnetic field, are caused to travel small cycloidal paths along the racetrack and result in the formation of additional ions as the electrons interact with the sputtering plasma.
While sputtering devices of the foregoing type are well-known to the art and widely used in certain industrial processes for coating substrates, several drawbacks reside therein which render them not altogether satisfactory for efficiently coating substrates susceptible to heat-induced deterioration. Substrates of this type include, for example, thin film substrates formed from polyester and other similar heat-sensitive materials.
Three sources of heating have been identified which contribute to the deterioration of substrates formed from polyester and other similar materials. The first of these sources, of course, are the sputtered atoms themselves which are emitted from the target for eventual deposition upon the substrate. These atoms arrive at the substrate with a substantial amount of kinetic energy that is converted to heat as the atoms impact the substrate surface being coated. Pre-cooling of the substrate prior to atom impaction therewith is a common method for mitigating this problem.
The second heat source is the fast electrons which leave the target at or near the potential of the ion bombardment energy. These fast electrons, which leave the target from surface portions thereof lying outside of the racetrack defined by the magnetic field, are not to any substantial degree influenced by the magnetic field and are, therefore, available for impaction upon the substrate. The prior art has collected some of these fast electrons by placing an anode about the perimeter of the target. While useful in collecting some of the fast electrons, the prior art solution does not solve the problem of collecting fast electrons emitted from the surface portion of the target which is surrounded by the racetrack.
And finally, the third identified heat source is the heated plasma itself. While the magnetic field of a planar magnetron maintains a fairly close confining force on the ions of the sputtering discharge or plasma, the mass of the ions is such that even at a few tens of electron volts those ions can escape the magnetic field. The ions which extend beyond the magnetic field exhibit a tendency to migrate towards the substrate and carry with them substantial quantities of both kinetic and potential energy, which energy is converted into heat by the impaction of those ions upon the substrate. If the plasma density is low, i.e.--in the microampere range, the ion heating effect is usually almost negligible. However, as the plasma density is increased, i.e.--in the milliampere range, a substantial amount of heating is experienced at the substrate. Insofar as the inventors herein are aware, no solution to the ion migration problem just described has been forthcoming and, until the present invention, is a problem which remains and detracts from the ability of presently available sputtering devices to coat heat-sensitive substrates.
In view of the foregoing, it is the primary object of this invention to provide new and useful means for confining heated plasma so that heat-sensitive substrates can be coated in a fast, efficient, reliable, and economical manner.
Another object of this invention is to provide new and useful means for intercepting and collecting fast electrons which would otherwise result in the deterioration of a heat-sensitive substrate.